Method for starting a continuous steam generator and continuous steam generator for carrying out said method

ABSTRACT

The invention relates to a continuous steam generator provided with an evaporator through flow heating surface which is disposed in a heating gas channel which can be cross flown in an approximately vertical manner in a heating gas device, said evaporator through flow heating surface comprising a plurality of parallel connected steam generating pipes enabling a flow medium to flow through, and an overheating heating surface which is arranged downstream from the evaporator through flow heating surface comprising a plurality of parallel connected overheating pipes enabling the evaporated flow medium to flow through, also enabling production and operational costs to be reduced and enabling the temperature of the steam on the outlet of the overheating heating surface to be controlled in a comparatively simple and flexible manner. The steam end-point of the flow medium is displaced to the overheating pipe, if required. The continuous heating surfaces and the overheating heating surfaces are combined to form one functional unit such that the overheating heating surface can be used as a steam heating surface in a continuous steam generator which is particularly suitable for carrying out said method.

CROSS REFERENCE TO RELATED APPLICATION

This application is the US National Stage of International ApplicationNo. PCT/EP2004/008655, filed Aug. 02, 2004 and claims the benefitthereof. The International Application claims the benefits of EuropeanPatent application No. 03020020.8 EP filed Sep. 3, 2003, both of theapplications are incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The invention relates to a continuous-flow steam generator of thehorizontal type of construction, in which there are arranged, in aheating gas duct through which the flow can pass in an approximatelyhorizontal heating gas direction, an evaporator continuous-flow heatingsurface, which comprises a number of steam generator tubes connected inparallel to allow a flow medium to flow through, and a superheaterheating surface which is connected downstream of the evaporatorcontinuous-flow heating surface and which comprises a number ofsuperheater tubes connected in parallel to allow the evaporated flowmedium to flow through. The invention further relates to a method forstarting a continuous-flow steam generator of this type.

BACKGROUND OF THE INVENTION

A steam generator of said type is known for example from EP 1 288 567A1.

In a gas and steam turbine plant, the heat contained in the expandedworking medium or heating gas from the gas turbine is utilized for thegeneration of steam for the steam turbine. The heat is transmitted in awaste-heat steam generator which is connected downstream of the gasturbine and in which a number of heating surfaces for water preheating,for steam generation and for steam superheating are normally arranged.The heating surfaces are connected into the water/steam circuit of thesteam turbine. The water/steam circuit conventionally comprises aplurality of, for example three, pressure stages, and each pressurestage may have an evaporator heating surface.

Several alternative implementation concepts, more specificallyimplementation as a continuous-flow steam generator or implementation asa circulating-flow steam generator, come under consideration for thesteam generator connected downstream of the gas turbine as a waste-heatsteam generator on the heating gas side. In the case of acontinuous-flow steam generator, the heating of steam generator tubesprovided as evaporator tubes leads to an evaporation of the flow mediumin the steam generator tubes in a once-only pass. In contrast thereto,in the case of a natural or forced circulation steam generator, thecirculated water is only partially evaporated during a pass through theevaporator tubes. After the generated steam has been separated, thewater that did not evaporate during this process is supplied once againto the same evaporator tubes for further evaporation, the evaporatedfraction being replaced by the water newly supplied to the evaporationsystem.

In contrast to a natural or forced circulation steam generator, acontinuous-flow steam generator is not subject to any pressurelimitations, so that fresh steam pressures well above the criticalpressure of water (P_(cri)≈221 bar), where there are still only slightdensity differences between a liquid-like and a steam-like medium, arepossible. A high fresh steam pressure is conducive to high thermalefficiency and therefore low CO₂ emissions of a fossil-fired powerstation. Moreover, as compared with a circulating-flow steam generator,a continuous-flow steam generator has a simple type of construction andcan therefore be produced with a particularly low outlay. The use of asteam generator designed on the continuous-flow principle as awaste-heat steam generator of a gas and steam turbine plant is thereforeespecially beneficial for achieving a high overall efficiency of the gasand steam turbine plant along with a simple type of construction.

Particular advantages as regards the outlay in terms of production, butalso as regards maintenance work which is required are afforded by awaste-heat steam generator of the horizontal type of construction, inwhich the heating medium or heating gas, that is to say the exhaust gasfrom the gas turbine, is routed through the steam generator in anapproximately horizontal flow direction. Such a steam generator isknown, for example, from EP 0 944 801 B1.

For efficiency or emission reasons, when a steam generator is beingstarted it is desirable to have as short a startup time, as it is known,as possible, that is to say the time until full-load conditions and athermal equilibrium of the plant components, along with full heating,are reached. A gas turbine requires a comparatively short time for thestarting operation or for load change operations. The exhaust gas from agas turbine can thus reach a high temperature relatively quickly. Ashort startup time of a gas turbine is desirable because it results inthe starting losses occurring during the starting operation andconsequently the emissions of the gas turbine being kept low.

If, as is the case in gas and steam turbine plants, a steam turbine isconnected downstream of the gas turbine on the heating gas side, thewaste heat from the gas turbine is utilized as heating gas in the steamgenerator of the steam turbine. For technical reasons, in particular dueto the thermal inertia of its large masses, the steam turbine requires alonger startup time than the gas turbine and therefore predetermines thelower limit for the necessary startup times. Since the gas turbinetherefore “leads” the steam turbine, the waste heat from the gas turbinecannot be absorbed completely by the steam circuit of the steam turbineduring the startup operation of the gas and steam turbine plant. Forthis reason, during the operation of starting the gas and steam turbineplant the major part of the hot steam is usually routed past the steamturbine into the condenser via what is known as a bypass system. Duringthis operation the steam turbine is carefully warmed up by means of asmall portion of the steam flow so as to avoid high rates of temperaturechange which would lead to inadmissible material stresses. Only afterthis comparatively lengthy warming-up operation of the steam turbine canthe latter be acted upon by the full steam volume and make itscontribution to the overall power output of the plant. Consequently,only the gas turbine power output is available for a certain period oftime during a starting operation.

In order to keep this period of time particularly short or even avoid itcompletely if at all possible, the steam produced in the steam generatorcan be cooled before it leaves the steam generator to a temperaturelevel approximating to the material temperatures of the steam turbineplant. This cooling is the more complicated, the further the gas turbine“leads” the steam turbine during startup.

The cooling is typically accomplished using an injection cooling meanswithin the superheater heating surfaces connected downstream of thecontinuous-flow heating surfaces of the steam generator during thestartup operation of the gas and steam turbine plant. However, thetemperature of the steam when it emerges from the superheater can becontrolled only within certain limits with the aid of injection cooling,with the result that even with measures of this type a certainwarming-up time for the steam turbine still generally remains. Moreover,the use of the separate cooling device is technically complex.

SUMMARY OF THE INVENTION

The object on which the invention is based is therefore to specify amethod for starting a continuous-flow steam generator of theabovementioned type, by means of which, using comparatively simplemeans, the temperature of the steam emerging from the tubes of thesuperheater heating surface can be reliably controlled and comparativelyshort startup times can be achieved. A continuous-flow steam generatorof the abovementioned type which is particularly suitable for performingthe method will also be specified.

With regard to the method, this object is achieved according to theinvention in that the evaporation end point of the flow medium istemporarily shifted into the superheater tubes.

In this case the invention proceeds from the consideration that in orderto reduce the technical overhead in terms of the assembly and operationof the steam generator, the cooling of the superheating heating surfacethat is necessary to enable short startup times of the gas turbineshould take place in a particularly simple way. Separate cooling devicessuch as, for example, injection coolers should therefore be avoided asfar as possible. A possible way to save investing in separate coolingdevices is the following: in order to avoid an excessive heating of thesteam, a portion of the flow medium that has not yet evaporated afterpassage through the evaporator heating surface and is therefore still inthe liquid state is provided to flow through the superheater heatingsurfaces. For this purpose, a water/steam mixture should be introducedinto the tubes of the superheater heating surface, which can be achievedby means of an increased feed water supply. In order to allow this, theevaporator heating surface and the superheater heating surface should becombined into a functional unit. This makes it possible to have a directflow of the liquid-medium/steam mixture out of the evaporator tubes overinto the superheater tubes. The evaporation end point for the flowmedium is thus shifted, as required, into the tubes of the superheaterheating surface.

The temperature of the steam supplied to the steam turbine at the exitof the superheater heating surface can in this case advantageously becontrolled directly via the feed water flow. This makes it possible toensure, for example during the starting operation or during a loadchange of the gas and steam turbine plant, that there is, within thesteam generator tubes of the superheater heating surface, sufficientliquid medium which, without a rise in temperature, absorbs heat throughevaporation and consequently reduces the superheating of the steam atthe exit of the superheater heating surface. By contrast, during thenormal operation of the plant, when the temperatures of the metal massesof the steam turbine are assimilated to the high steam temperatures, thelow temperature of the steam is not required and it is sufficient toplace the evaporation end point of the flow medium at the exit of theevaporator continuous-flow heating surface, for example. This enablesthe steam temperature at the exit of the steam generator to be adjustedin a particularly simple and at the same time highly flexible manner tothe operating state of the steam turbine.

The position of the evaporation end point within the superheater heatingsurface or the evaporator heating surface is beneficially controlled viathe amount of supply of flow medium per unit time. In this way theevaporation end point can be coordinated with the temperaturerequirements of the steam turbine in a particularly simple and flexibleway. To achieve low steam temperatures, an increased supply of flowmedium can be used, for example during the startup operation of the gasand steam turbine plant, to increase the proportion of flow medium notyet evaporated within the superheater heating surface quickly andwithout additional devices for the cooling of initially highlysuperheated steam.

With regard to the continuous-flow steam generator, the object isachieved according to the invention in that the evaporator heatingsurface and the superheater heating surface of the steam generator areinterconnected into a functional unit in such a way that the evaporationend point of the flow medium can be displaced into the superheaterheating surface.

The use, as and when required, of the superheater heating surface as anevaporator heating surface ensures the particularly flexible anduncomplicated operation of the steam generator in different operatingstates of the gas and steam turbine plant. During normal operation ofthe gas and steam turbine plant it is not necessary and, for reasons ofefficiency, not even desirable to utilize the superheater heatingsurface of the steam generator as an evaporator heating surface. Rather,the steam generator should be designed in such a way that the flowmedium has already evaporated completely at the exit of the evaporatorheating surface in order subsequently to be superheated in the tubes ofthe superheater heating surface downstream of the evaporator heatingsurface on the flow medium side. During the startup operation of the gasand steam turbine plant, on the other hand, it is desirable forunevaporated flow medium to pass into the superheater and evaporatethere, that is to say to absorb latent heat, and in so doing lower thetemperature of the steam at the exit of the superheater heating surface.The interconnection, provided for this purpose, of the evaporatorcontinuous-flow heating surface and the superheater heating surface onthe flow medium side is in this case preferably implemented byconsciously dispensing with an interconnection of the water separatortypically provided between the evaporator heating surface and thesuperheater heating surface.

Because the continuous-flow heating surface and the superheater heatingsurface are combined into one unit, there is no longer any need to use acommon outlet header for the steam flows from the parallel tubes, on theheating gas side, of a tube row of the evaporator heating surface andfor a redistribution of the flow to the parallel tubes of thesuperheater heating surface. In contrast it is preferably provided thatthe superheater tubes are preceded on the flow medium side in each caseby a number of individually assigned steam generator tubes in the mannerof individual sections connected in parallel on the flow medium side andwithout partial transverse communication, so that no redistribution ofthe flow medium at all is carried out between the evaporator heatingsurface and superheater heating surface. There is therefore also no riskof a segregation of the liquid and the steam phase of the flow medium.Overfeeding the evaporator, that is to say increasing the supply of flowmedium such that the flow medium cannot evaporate completely within thetubes of the evaporator heating surface, and transferring theliquid-medium/steam mixture out of the tubes of the evaporator heatingsurface into those of the superheater heating surface are consequentlypossible without difficulty and can thus be used to lower, as and whenrequired, the steam temperatures at the exit of the superheater heatingsurface during startup or during load changes.

The convergence of the flow from steam generator tubes connected inparallel on the flow medium side and arranged one behind the other onthe heating gas side in the manner of a tube line and the transfer intothe superheated tubes beneficially take place in each case by means of asuitably designed header/distributor unit, whereby a common headeroriented with its longitudinal axis essentially parallel to the heatinggas direction is in each case connected downstream of steam generatortubes connected in parallel on the flow medium side and arranged onebehind the other on the heating gas side. In this case the number ofheaders is conveniently equal to the number of steam generator tubesarranged within a tube row extending transversely to the heating gasdirection, so that each steam generator tube within a tube row isuniquely assigned precisely one header.

A separator is advantageously connected downstream of the superheaterheating surface on the flow medium side. The separator ensures that flowmedium which may not yet have evaporated, that is to say is still liquideven after passing through the superheater heating surface, cannot passinto the steam turbine.

A particularly high degree of flow stability and a particularlyfavorable heating profile can be achieved with only a small amount ofoverhead in structural and design terms in that provision is made forthe steam generator tubes of the continuous-flow heating surface to beadvantageously subdivided into in each case at least three segments (ofparallel tubes), the first segment of each tube comprising a rising tubepiece and having the flow passing through it in the upward direction.Analogously, the second segment comprises a falling tube piece and hasthe flow passing through it in a downward direction. In this case thefalling tube pieces of each steam generator tube which form the secondsegment are arranged in the heating gas duct in each case downstream ofthe rising tube pieces assigned to them, as seen in the heating gasdirection. The third segment comprises further rising tube pieces andhas the flow passing through it in the upward direction.

In this case the segments of the steam generator tube or of each steamgenerator tube are advantageously positioned in the heating gas duct insuch a way that the heating requirement of each individual segment ismatched to a special degree to the local heat availability in theheating gas duct. For this purpose, the further rising tube pieces ofeach steam generator tube which form the third segment arranged in theheating gas duct in each case between the rising tube pieces of thefirst segment which are assigned to them and the falling tube pieces ofthe second segment which are assigned to them, as seen in the heatinggas direction. In an arrangement of this type, therefore, the in eachcase first rising tube piece, which serves for partial preheating andlargely already for the evaporation of the flow medium, is exposed tocomparatively high heating by the heating gas in the “hot smoke gasregion”. This ensures that flow medium flows out of the respective firstrising tube piece with a comparatively high steam content in the entireload range. The result of this, during subsequent introduction into thedownstream falling tube piece, is that a rise of steam bubbles counterto the flow direction of the flow medium, said rise being unfavorablefor flow stability, is consistently avoided in the falling tube piece.Due to the arrangement of the falling tube piece in the “cold smoke gasregion” and to the arrangement of the further rising tube piece betweenthe first rising tube piece and the falling tube piece, an especiallyhigh efficiency of the evaporator heating surfaces is therefore ensuredby the high flow stability achieved in this way.

The steam generator is beneficially used as a waste-heat steam generatorof a gas and steam turbine plant. In this case a gas turbine isadvantageously connected downstream of the steam generator on theheating gas side. In this connection arrangement, additional firing forincreasing the heating gas temperature may beneficially be provideddownstream of the gas turbine.

The advantages achieved by means of the invention are in particular thatthe utilization of the actual flow medium makes it possible in aparticularly simple and technically uncomplicated way to adjust thetemperature of the steam at the exit of the superheater heating surfaceparticularly flexibly to the operating state of the steam turbine duringthe startup operation, so that the waiting time until the steam turbineis acted upon by steam for power output and the associated delay inpower output during starting can be kept particularly low in anespecially simple way.

It is particularly advantageous in this case to dispense with complexseparate cooling devices such as, for example, an injection coolingmeans. The use of the liquid portion of the flow medium and its capacityto absorb latent heat make it possible in an especially flexible andsimple way to control and, where required, to lower the temperature ofthe steam at the exit of the superheater heating surface. At the sametime the cooling of hot steam carried out during injection cooling,together with subsequent reheating, is no longer required.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention is explained in more detailwith reference to a drawing, in which:

FIG. 1 shows, in longitudinal section, a simplified illustration of acontinuous-flow steam generator of the horizontal type of construction,and

FIG. 2 shows a longitudinal section through a header of thecontinuous-flow steam generator.

DETAILED DESCRIPTION OF THE INVENTION

The continuous-flow steam generator 1 according to FIG. 1 is connecteddownstream of a gas turbine (not shown in any more detail) on theexhaust gas side in the manner of a waste-heat steam generator. Thecontinuous-flow steam generator 1 has a containment wall 2 which forms aheating gas duct 6 through which the flow is capable of flowing in anapproximately horizontal heating gas direction x, indicated by thearrows 4, for the exhaust gas from the gas turbine. A number of heatingsurfaces, configured on the continuous-flow principle, for thepreheating, evaporation and superheating of the flow medium are arrangedin each case in the heating gas duct 6. In the exemplary embodimentaccording to FIG. 1, only one evaporator continuous-flow heating surface8 is shown for the evaporation section, but a larger number ofcontinuous-flow heating surfaces may also be provided.

The evaporator system formed from the evaporator continuous-flow heatingsurface 8 can be acted upon by flow medium W which, during normal loadoperation, is evaporated completely during a once-only pass through theevaporator continuous-flow heating surface 8.

The evaporator system formed from the evaporator continuous-flow heatingsurface 8 is connected into the water/steam circuit (not shown in anymore detail) of a steam turbine.

The evaporator continuous-flow heating surface 8 of the continuous-flowsteam generator 1 according to FIG. 1 comprises, in the manner of a tubegroup, a plurality of steam generator tubes 12 connected in parallel toallow the flow medium W to flow through. In this arrangement, aplurality of steam generator tubes 12 are in each case disposed next toone another, as seen in the heating gas direction x, such that what areknown as tube rows are formed. Thus, in each case only one of the steamgenerator tubes 12 arranged next to one another in this way can be seenin FIG. 1. A plurality of steam generator tubes 12 are arranged onebehind the other, as seen in the heating gas direction x, andconsequently form what is known as a tube line. A common header 16 isconnected here downstream of the steam generator tubes 12 arranged in atube line in each case on the flow medium side.

A superheater heating surface 20, likewise arranged in the heating gasduct 6, is connected downstream of the evaporator continuous-flowheating surface 8 on the flow medium side. In addition to the evaporatorsystem and the superheater heating surface 20, the water/steam circuitof the steam turbine has connected into it a number of further heatingsurfaces (not shown in FIG. 1) which may be, for example,medium-pressure evaporators, low-pressure evaporators and/or preheaters.

The continuous-flow steam generator 1 is designed for reliably ensuring,by particularly simple means, low steam temperatures at the exit of thesuperheater tubes 22 forming the superheater heating surface 20 duringthe startup operation. For this purpose there is provision to shift theevaporation end point of the flow medium W as required from the steamgenerator tubes 12 into the superheater tubes 22, so that, owing to theresidual evaporation in the superheater tubes 22, particularly in thestartup or part-load range, particularly low superheating can beachieved by suitable setting of the feed of flow medium W to the steamgenerator tubes 12.

For this purpose the headers 16 are designed in such a way that theyallow the completely or partially evaporated flow medium W to flow overinto the superheater heating surface 20 connected downstream of theevaporator continuous-flow heating surface 8, without the liquid phaseand the steam phase of the flow medium W being segregated in theprocess. The header/distributor unit thus connects the evaporatorcontinuous-flow heating surface 8 and the superheater heating surface 20into a functional unit in such a way that the evaporation end point ofthe flow medium W can be displaced into the superheater heating surface20.

During normal operation, the steam D generated in the evaporator unitfrom the flow medium W is superheated in the superheater tubes 22forming the superheater heating surface 20. The number of superheatertubes 22 arranged next to one another in the heating gas direction xcorresponds to the number of tube lines of the evaporator system. Thesteam D generated in the steam generator tubes 12 of a tube line istherefore collected in each case in a header 16 and supplied in eachcase to one or two superheater tubes 22, so that each superheater tube22 is preceded on the flow medium side in each case by a number ofindividually assigned steam generator tubes 12. The direct flow,produced by this arrangement, out of the evaporator system over into thesuperheater tubes 22, without a separator connected between theevaporator system and superheater heating surface 20, allows thecontrolled transfer of not yet evaporated, that is to say still liquidflow medium W out of the evaporator system into the superheater tubes22. Owing to the shift, realized in this way, of the evaporation endpoint of the flow medium W out of the evaporator system into thesuperheater tubes 22, it is possible to achieve a lowering of thetemperature of the steam D at the exit 24 of the superheater tubes 22which is desired depending on the operating state of the gas and steamturbine plant. This takes place due to the evaporation of the liquidmedium portion in the superheater tubes 22, that is to say, the use, asand when required, of the superheater heating surface 20 as anevaporator heating surface.

Particularly during startup or load change operations of the gas andsteam turbine plant, a lowering of the temperature of the steam D at theexit 24 of the superheater tubes 22 is required, since the steamturbine, due to its sluggish behavior compared with that of the gasturbine, does not allow the steam temperatures to follow the exhaust-gastemperatures quickly during starting. By the superheater heating surface20 being utilized as required as an evaporator heating surface, thetemperature of the steam D at the exit 24 of the superheater heatingsurface 20 can be adjusted in an especially simple and flexible way tothe lower temperature of the metal masses of the steam turbine, that isto say can be lowered.

An especially flexible setting of the steam temperature T at the exit 24of the superheater heating surface 20 is ensured in that the evaporationend point can be displaced within the superheater tubes 22 in the flowdirection y, identified by the arrow 26, of the flow medium W. Anevaporation end point lying comparatively far forward in the flowdirection y, that is to say in the vicinity of the headers 16, signifiesa low heat absorption capacity of the liquid-medium/steam mixture withinthe superheater heating surface 20 and consequently a comparatively hightemperature T of the steam D at the exit 24 of the superheater heatingsurface 20. If, however, the evaporation end point is displacedcomparatively far into the superheater tubes 24 in the flow direction y,that is to say the flow medium W evaporates completely onlycomparatively late, then the heat absorption capacity of theliquid-medium/steam mixture within the superheater heating surface 20 ishigh and the temperature T of the steam D at the exit 24 of thesuperheater heating surface 20 is comparatively low.

In the exemplary embodiment, the position of the evaporation end pointin the flow direction y and consequently the temperature T of the steamD at the exit 24 of the superheater heating surface 20 are controlledvia the supply of flow medium W to the steam generator tubes 12, that isto say via the feed water stream. For this purpose, the pump power ofthe feed water pumps is activated accordingly by a central monitoringand control unit. When a comparatively large quantity of flow medium Wper unit time is supplied to the steam generator tubes 12, the amount ofheat made available by the heating gas is not sufficient to evaporatethe flow medium W completely within the evaporator continuous-flowheating surface 8. Thus, the greater the quantity of flow medium Wsupplied per unit time to the steam generator tubes 12, the higher isthe liquid medium portion in the liquid-medium/steam mixture whichpasses out of the evaporator system into the superheater tubes 22 viathe header/distributor unit. A high liquid medium portion in turnrequires a comparatively high heat absorption capacity of theliquid-medium/steam mixture and a comparatively low exit temperature T.Thus, in an especially simple and flexible way, a lowering of thetemperature T of the steam D supplied to the steam turbine can beachieved solely by an increase in the supply of flow medium W per unittime and, conversely, an increase in the temperature T can be achievedsolely by a lowering of the supply.

Furthermore, the evaporator continuous-flow heating surface 8 isdesigned for an especially favorable heating characteristic. In order toensure this in an especially reliable way by particularly simplestructural means, the evaporator continuous-flow heating surface 8comprises three segments connected in series on the flow medium side. Inthe first segment, each steam generator tube 12 of the evaporatorcontinuous-flow heating surface 8 in this case comprises anapproximately vertically arranged rising tube piece 28 through which theflow medium W is capable of flowing in the upward direction. In thesecond segment, each steam generator tube 12 comprises an approximatelyvertically arranged falling tube piece 30 which is connected downstreamof the rising tube piece 28 on the flow medium side and through whichthe flow medium W is capable of flowing in the downward direction. Inthe third segment, each steam generator tube 12 comprises anapproximately vertically arranged further rising tube piece 32 which isconnected downstream of the falling tube piece 30 on the flow mediumside and through which the flow medium W is capable of flowing in theupward direction. The falling tube piece 30 is in this case connected tothe rising tube piece 28 assigned to it via an overflow piece 34. In thesame way, the further rising tube piece 32 is connected to the fallingtube piece 30 assigned to it via an overflow piece 34. Viewed in theheating gas direction x, the further rising tube piece 32 is arrangedbetween the rising tube piece 28 and the falling tube piece 30.

The shift of the evaporation end point of the flow medium W out of theevaporator system into the superheater tubes 22 is made possible by theuse of the headers 16 illustrated in more detail in FIG. 2.Conventionally, the flow medium W evaporated for the most part at theexit of the evaporator continuous-flow heating surface 8 is collected inan outlet header and redistributed by a distributor to the superheatertubes 22 connected downstream of the steam generator tubes 12. However,the use of a common header for the steam generator tubes 12 of a tuberow and the resulting requirement for a redistribution of the flowmedium W to the superheater tubes 22 give rise to the risk of anundesirable segregation of the liquid and the steam phase. If, on theother hand, as in the exemplary embodiment, common headers anddistributors for steam generator tubes 12 of a tube row are dispensedwith and, in their place, only one header 16 is used for steam generatortubes 12 of a tube line, this risk is no longer present. Theliquid-medium/steam mixture flows without segregation out of the steamgenerator tubes 12 of a tube line into the header 16 and from there intothe following superheater tube 22, without a redistribution of the flowmedium W being necessary. The separator 36 usually connected between theevaporator system and the superheater heating surface 20 is placed atthe exit 24 of the superheater tubes 22.

1. A method for starting a power plant of the type having a gas turbine,a steam turbine and a continuous-flow steam generator, comprising:positioning the steam turbine downstream of the gas turbine and applyingheat exhausted from the gas turbine as a heating gas in the steamgenerator by: (i) providing an evaporator continuous-flow heatingsurface that is arranged in a heating gas duct through which the flow iscapable of flowing in an approximately horizontal heating gas direction,and which comprises a number of steam generator tubes connected inparallel to allow a flow medium to flow through; and (ii) providing asuperheater heating surface that is connected downstream of theevaporator continuous-flow heating surface on the flow medium side andwhich comprises a number of superheater tubes connected in parallel toallow the evaporated flow medium to flow through, and (iii) positioningthe evaporation end point of the flow medium in the superheater tubes totemporarily limit the temperature of the flow medium at an exit of thesuperheater heating surface while starting up the steam turbine.
 2. Themethod as claimed in claim 1, wherein the temperature of the flow mediumat an exit of the superheater heating surface is controlled by theselection of the position of the evaporation end point of the flowmedium in the superheater heating surface.
 3. The method as claimed inclaim 1, wherein the evaporation end point of the flow medium is set viathe feed rate of the flow medium supplied to the evaporatorcontinuous-flow heating surface.
 4. A power plant comprising a gasgenerator, continuous-flow steam generator coupled to generate steamwith heat exhausted by the gas turbine and a steam turbine coupled toreceive the generated steam, the steam generator comprising: a heatinggas duct arranged to provide a flow in an approximately horizontalheating gas direction; an evaporator continuous-flow heating surfacethat comprises a number of steam generator tubes connected in parallelto allow a flow medium to flow therethrough; and a superheater heatingsurface connected downstream of the evaporator continuous-flow heatingsurface and which comprises a number of superheater tubes connected inparallel to allow the evaporated flow medium to flow therethrough,wherein the evaporator continuous-flow heating surface and thesuperheater heating surface are interconnected into a functional unit insuch a way that the evaporation end point of the flow medium can bedisplaced into the superheater heating surface.
 5. The power plant ofclaim 4, wherein each superheater tube is preceded on the flow mediumside in each case by a number of individually assigned steam generatortubes.
 6. The power plant of claim 4, wherein a common header orientedwith its longitudinal axis essentially parallel to the heating gasdirection is in each case connected downstream of steam generator tubesconnected in parallel on the flow medium side and arranged one behindthe other on the heating gas side.
 7. The power plant of claim 4,wherein the number of headers is equal to the number of steam generatortubes arranged within a tube row extending transversely to the heatinggas direction.
 8. The power plant of claim 4, wherein a separator isconnected downstream of the superheater heating surface on the flowmedium side.
 9. The power plant of claim 4, wherein a steam generatortube comprises an approximately vertically arranged rising tube piecethrough which the flow medium is capable of flowing in the upwarddirection and an approximately vertically arranged falling tube piecethat is connected downstream of said rising tube piece on the flowmedium side and in the heating gas direction and through which the flowmedium is capable of flowing in the downward direction, and anapproximately vertically arranged further rising tube piece which isconnected downstream of said falling tube piece on the flow medium sideand through which the flow medium is capable of flowing in the upwarddirection and that is arranged between the rising tube piece and thefalling tube piece as seen in the heating gas direction.
 10. The powerplant of claim 4, wherein the continuous-flow steam generator ispreceded on the heating gas side by the gas turbine.